CAS systems capable of real time location and tracking of a plurality of discrete objects in a surgical field are now becoming increasingly well known. A variety of systems are employed, however all require the patient bone elements to be identified and registered to pre-operatively taken anatomical scans or intra-operatively taken images of the same bone elements. In order for the relevant bone elements to be located and tracked by the CAS system, trackable reference members must be fastened thereto. These bone reference members will vary depending on the type and specific requirement of the particular CAS system used.
For example, for an optical CAS system, the trackable bone reference members will comprise at least three optically detectable markers whose exact position can be determined by each of the at least two cameras of the optical CAS system. This therefore permits the position in space of each detectable marker to be determined by the CAS system, and therefore permits the position and orientation of the bone reference member, and consequently also the position and orientation of the bone element to which it is affixed, to be determinable by the CAS system.
However, installation of traditional bone reference members, which are identifiable by the CAS system and are fixed to each bone element, can often be significantly invasive to the patient. Relatively large incisions are sometimes required in order to expose a section of bone sufficiently large enough to accommodate the bone reference that is to be fastened thereto. Any unnecessary invasiveness can result in significant additional patient trauma and post-operative pain. As such, bone reference blocks that can be fastened to a bone element with a minimum amount of invasiveness have been more recently developed. For example, rather than having a reference block screwed directly to an exposed portion of a bone element, only positioning pins, inserted through relatively smaller incisions in the skin, are driven into the bone element. These pins are therefore anchored in the bone, and extend beyond the skin such that a reference block can be engaged to the exposed portions of the pins. This thereby permits reference members to be indirectly fixed to the bone elements via the pins which can be fastened to the bone element with a limited amount of invasiveness.
However, for certain particular applications, it can be necessary to use a bone reference member that is directly fastened to the bone element. Also, some surgical uses may not permit the obstructed space required for the relatively wider stance that the pined-type bone reference members occupy.
It is currently known in total hip replacement surgery to use a CAS femoral reference member that is elongated and comprises a toothed proximal end for direct engagement with the femur. The elongated body of the reference member is hollow, such that once the toothed end is engaged with the bone surface, a long cortical screw, such as a Trochar screw for example, can be inserted through the body of the reference member and fastened into the bone. A flange on the distal end of the screw serves to engage the distal end of the reference member and forces the teeth of the toothed proximal end thereof into the bone surface. The reference member is thereby axially and rotatably fixed relative to the bone element. A trackable member as described above is often integrally formed with the distal end of the reference member, thereby permitting the CAS system to determine the position and orientation of the bone reference member, and therefore track the movement of the bone element to which the reference member is fixed.
However, the installation of such a bone reference member also causes significant damage to soft tissue. Although it requires a smaller incision than the larger blocks that are fastened directly to the bone surface, the sharp teeth at the proximal end of the elongated body of such a bone reference member tend to cut and tear the surrounding soft tissue as the proximal end is inserted therethrough until it contacts the bone.